For expedient function, electronic-equipment must, usually be shielded from electromagnetic radiation. The equipment may also comprise components which themselves generate electromagnetic radiation that must be shielded.
To provide such shielding, the electronic equipment, or its components, is normally enclosed in casings with electrical conductivity, which consequently act as a Faraday cage.
To allow access to the electronic equipment or its components, these casings are usually made to be opened, in which case elastic elements of the type described by way of introduction are used to ensure the necessary shielding.
Such an element is known from, for example, GB 2049718. The element described comprises an elastic, electrically non-conductive material which carries electrically conductive flakes. The flakes are oriented to increase the conductivity of the element in a certain direction. To achieve this orientation, the material, when still viscous, is subjected to a shearing process, which can be effected by extrusion. Subsequently, the material is allowed to cure, after which the material is sliced in a direction which is preferably perpendicular to the direction of extrusion. The completed element is finally punched from the slices of material. Although the thus manufactured elements for electromagnetic shielding have an advantageous conductivity in a desired direction, they are difficult to manufacture and besides it is difficult to provide more complicated designs of the elements.
Moreover, a casing with an elastic element of the type described by way of introduction is known from, for example, U.S. Pat. No. 5,882,729.
The element described in the above publication is manufactured by dispensing a viscous material carrying particles with substantial electrical conductivity on a housing. The viscous material is dispensed in the form of a bead of the required extension, after which the material is treated to assume an elastic, non-viscous state. The element ensures good electric contact between the housing and a cover when this is closed and caused to abut against the element.
A problem with elements of this type is that the particles that provide the electrical conductivity of the element are relatively expensive. It would therefore be desirable to reduce the amount of particles which is included in the element.
Furthermore, for dispensing to be possible, the material must have a relatively low viscosity. As a result, the dispensed bead will have a shape corresponding to a lying D.
A thus designed element requires a relatively high compression force to achieve the necessary electric contact between, for instance, a housing and a cover.
In many fields, it is required that the electronic equipment be made increasingly smaller. For instance, there is an ongoing development towards manufacture of smaller, thinner and lighter mobile phones. Unnecessarily high compression forces may in this context cause deformation of the casing of the mobile phone.
Also in shielding covers for base stations for mobile telephony there is a need for lowered compression forces in shielding elements, since the now prevailing relatively high compression forces require expensive stiffeners and/or great wall thicknesses.
There is thus a need for elements that require lower compression forces. The solution suggested in U.S. Pat. No. 5,882,729 is dispensing of a plurality of beads, thereby building a vertically tapering element. It will be appreciated that this is a complicated and time-consuming process which has a detrimental effect on the cost of manufacture of the element.
A further method of making thus tapering elements is injection moulding, but this is not a practically applicable method for surface-sensitive or large components.
Finally it is known from U.S. Pat. No. 4,778,635 to make a material with anisotropic electrical conductivity by subjecting a viscous material, which carries electrically conductive particles, to a spatially varying magnetic field while at the same time the material cures. More specifically, the varying magnetic field affects the particles so that they are concentrated in beads whose positions are locked as the material cures.